You can specify the same data directory with "-datadir" in a .conf file. See here.
it is not for end-users
it is for professionals
Its not even a solution*, read the post by gmaxwell again. I think for those "end-users" you have in mind a slim client (e.g. Electrum, Multibit, etc.) would be perfect. While I dont think that its good to have less and less full nodes Im also positive that there will be enough "professionals" as you call them.
*IIRC its not even possible to define a data dir from config file, but only via launch argument. That information might be outdated though.
You can't specify the datadir in the config file, because the config file is stored in the datadir. The wiki page has specifics on what can and can't be specified in the config file.
